Effect of the anisotropic chain motion in molten polymers: The solidlike contribution of the nonzero average dipolar coupling to NMR signals. Theoretical description

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Abstract
A model of chain fluctuations analogous to that proposed by Rouse is used to describe NMR properties of molten polymers. As a general property of the magnetic relaxation of a single proton pair, it is shown that within the motional narrowing approximation, the nonzero average dipolar coupling gives rise to a free procession signal with two components in the rotating frame. One is in quadrature with the pulse radio‐frequency field, as is usual in ordinary liquids. However, its time evolution is typical of the presence of a nonzero average dipolar coupling. The other one is in phase with the rf field, as is usual in ordinary solids; its amplitude is proportional to the nonzero average of dipolar coupling within the proton pair. It is also shown that this component can be observed by using suitable pulse sequences already proposed to study spin systems in solids. In the same approximation, it is shown that the high resolution resonance spectrum of such a proton pair may be strongly modified in the presence of low‐speed rotation of the sample. These effects are exactly described. Furthermore, by taking into consideration the properties of the model of molten polymers presently chosen, the NMR signals are expressed as a function of the essential parameters of the polymer system. The observation of a signal in phase with the rf field is predicted to reflect closely the presence of permanent chain constraints exerted on the polymer chains in the molten system. Short numerical and experimental illustrations are given. It is shown that a signal in phase with the rf can be observed on molten cis ‐ 1,4‐polybutadiene at room temperature.